Make CompositorObserver’s destructor protected-and-virtual.

ui/compositor/compositor_observer.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_COMPOSITOR_COMPOSITOR_OBSERVER_H_
 #define UI_COMPOSITOR_COMPOSITOR_OBSERVER_H_
 
 #include "base/time/time.h"
 #include "ui/compositor/compositor_export.h"
 
 namespace ui {
 
 class Compositor;
 
 // A compositor observer is notified when compositing completes.
 class COMPOSITOR_EXPORT CompositorObserver {
  public:
+  virtual ~CompositorObserver() = default;
+
   // A commit proxies information from the main thread to the compositor
   // thread. It typically happens when some state changes that will require a
   // composite. In the multi-threaded case, many commits may happen between
   // two successive composites. In the single-threaded, a single commit
   // between two composites (just before the composite as part of the
   // composite cycle). If the compositor is locked, it will not send this
   // this signal.
   virtual void OnCompositingDidCommit(Compositor* compositor) = 0;
 
   // Called when compositing started: it has taken all the layer changes into
   // account and has issued the graphics commands.
   virtual void OnCompositingStarted(Compositor* compositor,
                                     base::TimeTicks start_time) = 0;
 
   // Called when compositing completes: the present to screen has completed.
   virtual void OnCompositingEnded(Compositor* compositor) = 0;
 
   // Called when the compositor lock state changes.
   virtual void OnCompositingLockStateChanged(Compositor* compositor) = 0;
 
   // Called at the top of the compositor's destructor, to give observers a
   // chance to remove themselves.
   virtual void OnCompositingShuttingDown(Compositor* compositor) = 0;
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_COMPOSITOR_OBSERVER_H_